Utility companies typically distribute power to customers using a network of power lines, cables, transformers, and switchgear. Distribution switchgear is medium voltage (e.g. 1 kV-38 kV) equipment used to control the flow of power and current through the distribution network by opening and closing under established criteria, for instance, tripping open when a damaging high-current fault occurs within the system. Distribution switchgear typically consists of a current interrupter, such as a vacuum interrupter, a mechanism to open and close the current interrupter, a sensing system to detect the status of the distribution network, and insulation encompassing some or all of these components. The sensing system may include a current sensor, a voltage sensor, or various other types of sensors.
Various exemplary vacuum interrupters, sometimes called vacuum bottles or vacuum tubes, are described in U.S. Pat. No. 8,450,630. One such exemplary vacuum fault interrupter 100 is shown in FIG. 1. A contact 102 is movable relative to a stationary contact 101. They are contained inside a sealed envelope consisting of an insulator 115, typically a ceramic, endcaps 111 and 112, and a flexible bellows 118, which allows the motion of the movable contact 102 on the same axis as the insulator 115 without loss of the seal. Air is removed from this envelope, leaving a deep vacuum 117, which has a high voltage withstand, and desirable current interruption abilities.
Current enters the vacuum interrupter through the stationary end connection 107. End connection 107 may be made from one or more pieces. Inside the vacuum interrupter, current is directed through a stationary coil segment 105, which has slots cut into it that force current to follow a substantially circumferential path before entering the stationary contact 101. Likewise, upon exiting the movable contact 102, current is again forced to follow a substantially circumferential path by slots cut into movable coil segment 106, before exiting the vacuum interrupter via moving end rod 108. End rod 108 may be constructed out of more than one piece. Current flow may also be reversed. There may also be one or more contact backings 103, 104, between the coil segments 105, 106 and the contacts 101, 102. Both the contact backings 103, 104, and the slots cut into the coil segments 105, 106, may be used to generate a magnetic field parallel to the main axis of the contacts 101, 102, and the insulator 115. The axial magnetic field may be used to control electrical arcing that occurs when the contacts are separated. Other arc control methods may be used as well. The end rods 107, 108, and the coil segments 105, 106 are typically made of copper. Reinforcing rods 109, 110, may be added to reinforce and strengthen the structure, and may be made of any applicable structural material such as stainless steel. One or more threads may be added at either end to facilitate either the electrical connection to the distribution network or the mechanical connection necessary to open the interrupter, for instance, threaded insert 119, which may be made out of any applicable structural material, such as stainless steel. Endcaps 111, 112 may also be shaped to protect any triple joints that may exist at either end of insulator 115 from high electrical stress. Alternately, separate end shields may be provided. Center shield 116 is also provided to grade electrical stress and protect insulator 115 from arcing that may occur when the contacts open. Center shield 116 may be mounted by being brazed to retaining ring 120 that sits in groove 121 in insulator 115.
An exemplary insulation system is shown in FIG. 2 (prior art). Insulation system 200 uses a modified vacuum interrupter 100′. Compared with vacuum interrupter 100, modified interrupter 100′ has a hollow moving rod 208 to accommodate a contact pressure spring 231 as described with respect to FIG. 12 of in U.S. Pat. No. 6,867,385. Contact pressure springs provide opening energy to operating mechanisms while also providing contact closing force and allowing for vacuum interrupter contact erosion. Contact pressure spring 231 is held in place with spring coupler 248 by pin 247. Vacuum interrupter 100′ has also been modified to add a piston 232 for holding a louvered contact band sliding style current exchange. This band slides along the inside diameter of current exchange housing 233. Other current exchanges may be used as well, for instance, the flexible wires shown in U.S. Pat. No. 5,597,992. The contacts of vacuum interrupter 100′ are shown as if open at full gap.
Vacuum interrupter 100′ is encapsulated in a solid dielectric 234, for instance epoxy. Buffer layer 235 may be used to absorb differences in the coefficient of thermal expansion between the insulator 115 of vacuum interrupter 100′ and the solid dielectric 234. Buffer layer 235 may be an expanded compliant material, as described in U.S. Pat. No. 5,917,167, for instance, silicone rubber. End conductors 236, 237 thread into the stationary end 107 of the vacuum interrupter and into the outside diameter of current exchange housing 233, respectively, to carry current into and out from vacuum interrupter 100′.
Current transformer 238 may wrap around end conductor 237, and may be mounted to base 240 via tube 239, as described in U.S. Pat. No. 6,760,206. Current transformer 238 is used to detect the amount of current flowing through end conductor 237 and vacuum interrupter 100′. The output wires from current transformer 237 may be routed through tube 239.
Operating rod 241 may be connected to contact pressure spring 231 and used to open and close vacuum interrupter 100′ by moving contact 102 relative to stationary contact 101 and base 240. While contact pressure spring 231 is shown nested inside the moving rod 208, it could also be embedded in operating rod 241 or be elsewhere in the mechanical system. Operating rod 241 may also contain one or more resistors 242 as part of a voltage sensor, as described in U.S. Pat. No. 7,473,863.
Solid dielectric 234 includes an operating cavity 243, which allows motion of operating rod 241 relative to base 240 by an operating mechanism (not shown). Cavity 243 is typically air filled, but may also be filled with other insulating fluids, for instance: mineral oil or sulfur hexalluoride (SF6). Insulating rubber plug 244 may increase the dielectric strength of cavity 243 by surrounding the open end of current exchange housing 233, as described in U.S. Pat. No. 6,828,521 and reducing discharges. Grading shield 245 may completely or partially surround cavity 243, and reduce electrical stress in cavity 243 as a result of a close proximity of grounded current transformer 238 and the high voltage end of operating rod 241, as described in U.S. Pat. No. 7,148,441. Drip sheds 246 may protect the operating cavity 243 from condensation, as described in U.S. Pat. No. 5,747,765.
Similarly, one or more horizontal sheds 251 or vertical sheds 252 may protect insulation system 200 from environmental influences, such as: condensation, pollution, arcing, or electrical creep. One or more horizontal sheds 251 or vertical sheds 252 may also serve to dissipate heat.
While insulation system 200 provides a robust method of insulating a vacuum interrupter and various sensors, there are disadvantages to the system.
Insulation system 200 is typically made by encapsulating epoxy resin around the various components, and then allowing the epoxy to cure and solidify. Voltage classes are predetermined based on the size of the mold: smaller molds are used for lower voltage classes and inserts are typically added to the mold to increase its size for higher voltage classes. Furthermore, the choice of vacuum interrupter type, conductor size, and current transformer type must also be made prior to encapsulation. Thus, once a specimen is molded, it is impossible to change voltage or current ratings, or any other options. Thus, insulation system 200 is not flexible per production demands.
Likewise, if damage occurs to any component, for instance: horizontal shed 251 is chipped, the entire insulation system 200 must be discarded, even if the remaining components are still in good condition. Insulation system 200 is not flexible per servicing demands.
Furthermore, while insulation system 200 allows detection of voltage at one of the two end conductors via operating rod 241 and resistor 242, it does not allow detection at the opposite end. A resistive or capacitive sensor passing from end conductor 236 would pass near vacuum interrupter 100′ and current exchange housing 233. This would result in a high electrical stress in insulation system 200, where two different voltages would pass by each other. Furthermore, a high amount of electrical cross-talk might then occur as a result of a capacitance coupling that may exist between the two voltages, resulting in a loss of accuracy of both voltage output signals.
It is desirable to provide an insulating system that would allow voltage and current ratings, as well as other options, to be determined after the insulation system is manufactured. It is desirable to have an insulating system that allows replacement of damaged components without discarding and replacing the entire system. It is also desirable to find an insulation system that would allow multiple voltage and current signals to be sensed, without high electrical stress or cross-talk.